1. Technical Field
The present invention relates to a semiconductor device and a method for fabricating the same, more particularly to a semiconductor device with symmetric epitaxial structures and a method for fabricating the same.
2. Description of the Related Art
With the development of the semiconductor technology, the speed of transistors is unceasingly increased. However, the speed of transistor is still limited to the mobility of electrons and holes in the silicon channels. Accordingly, one way to improve the device performance is to adjust the mechanical stresses of the channels and thereby raise the mobility of electrons and holes in the channels.
A prior-art method of adjusting the mechanical stress is to form a strained semiconductor material, such as silicon germanium alloy (SiGe), as the major material of source/drain (S/D) regions. The method includes removing portions of the substrate at the predetermined positions of the S/D regions to form trenches and then filling SiGe into the trenches with selective epitaxial growth (SEG). Because the effective electron mass and the effective hole mass are smaller in germanium than in silicon, the mobility of electrons and holes can be raised by forming the S/D regions mainly from SiGe. Thereby, the performance of the device can be improved.
However, if the epitaxial structures formed at two sides of the gate of a transistor are asymmetric, the transistor can not gain an appropriate stress effect therefrom.